Magnetic resonance imaging (MRI) scanners offer a noninvasive and highly accurate method for assessing many types of knee injuries. Most MRI scanners are large units consisting of a long, narrow tunnel and a patient tray. The patient lies on the tray while a technician positions the patient's knee. Once the knee position is set, the technician slides the patient tray into the tunnel. The unit then exposes the patient to a strong magnetic field while taking a series of high-quality images of the knee. Ideally, the knee images are taken in specific angular increments. A physician can then get a good idea of how the knee functions as it moves through a range of motion.
Despite the great promise of MRI in the diagnosis of knee injuries, certain practical problems have arisen in properly positioning the knee for a scan. To study the relationship of the patella or knee cap, the femur or upper leg bone, and the tibia and fibula, or lower leg bones, it is desirable to obtain views of the knee with the upper and lower leg at various angular orientations. Unfortunately, when a patient lies in a prone position, gravity tends to straighten the knee. Also, an unsupported knee tends to move around over time, thereby distorting the MRI images.
Attempts to maintain the knee in a bent position within the MRI scanner have been awkward. One common approach has been to simply prop-up the knee with pillows. This approach has several major problems. First, the knee tends to change position when the pillows shift. As noted above, shifting knee position distorts the MRI images. And second, the pillow approach does not offer any means to calibrate the precise angle of the knee. Thus, a physician cannot know what angular knee position a certain image corresponds to, thereby detracting from the physician's ability to make a proper diagnosis. Further, when the angle of the knee is to be changed, the patient must be slid out of the MRI chamber, re-oriented and then slid back into the MRI apparatus, with the procedure taking much longer than would be desired, with corresponding annoyance by the patient and prolonged use of the expensive MRI apparatus.
Another approach is to employ an awkward board-like apparatus that runs the length of the patient's body. The patient lies down prone on the board, which sits atop the patient tray. The board is hinged at about the point of the patient's knee, and the entire portion of the board distal to the patient's knees rotates to incline upward. A long lever is sometimes provided to raise or lower the hinged portion of the board.
Like the pillow approach, the board apparatus has a number of drawbacks. The apparatus does not provide set vertical positions, so exact calibration of knee angle is not practical. Storage of the apparatus is also a problem. Since the apparatus is so large, it must be removed from the MRI room when not in use. Additionally, the apparatus is clumsy and inconvenient to use.